1. Field of the Invention
The present invention relates to a method for manufacturing a circuit board, and particularly to a method for manufacturing a circuit board provided with a so called through hole.
2. Prior Art
Circuit boards used for electronic apparatus nowadays are frequently provied with through hole in compliance with the demand for high density assembly of electronic parts. In response to such trend, various methods for manufacturing circuit boards, with purpose of improving the reliability in continuity of through hole have been proposed. Of those methods provided, from the viewpoint of the reliability of the through hole, so called the CC-4 method is considered to be highly recommendable as manufacturing method. However, this method cannot necessarily be regarded preferable in terms of mass production, because of its defect that it takes as long as 20-30 hours for forming the metal layer of about 35.mu. thick.
Therefore, even now, many circuit boards having the through holes are manufactured by the methods as continuation of the conventionally used processes which are called the etched foil process or the subtractive process.
FIG. 1, (A) and (B) respectively show the processes manufacturing the circuit boards with through holes, by using such methods as mentioned above. In FIG. 1, (A) shows an example of manufacturing processes by using a copper-clad laminate, while (B) in the same Figure shows an example wherein copper is precipitated and grown on plastic board for manufacturing.
In (A) of FIG. 1, (b) shows the state wherein a pierced hole 3 is formed to be used as the through hole, at a desired position in a copper-clad laminate (a) with the specified dimensions. Designated at 1 is a board made of epoxy material or phenol material, and 2 is about 35.mu. thick copper foil adhered in advance onto the above-mentioned board 1. After providing the through hole 3, its surface is pretreated for surface activation, by means of chemical blocking and palladium solution, etc. Thereafter, the electroless plating of copper is performed ((c) of FIG. 1 (A)). Then, by using the electroplating, about 35.mu. thick plating copper 5 is precipitated and grown over the copper foil 2 as well as over the copper plate film 4 that is formed by the electroless plating over the surface of the through hole 3 ((d) of FIG. 1 (A)). Furthermore, the wiring section is masked with the plating resist film 6 obtained by a technique of the screen printing of the epoxy system ink ((e) of FIG. 1 (A)). Finally, through removing the unmasked portion with etching solution, then by removing the foregoing plating resist film 6, the circuit board having the through hole with desired wiring pattern can be obtained.
In the method shown in (B) of FIG. 1, the pierced hole 3 to be used as through hole is formed in the board with a specified size in the processing stages of (a) and (b), then, the copper plate film 4 is formed over the entire surface of the board, including the surface of the through hole 3, by the electroless plating, in the process (c). Thereafter, by taking the respective processing steps of the formation of the plating resist film 6 ((d) of FIG. 1 (B)), the formation of the plated copper for the wiring section by electroplating ((a) of FIG. 1 (B)), and the etching ((f) of FIG. 1 (B)), the circuit board having the through holes with the desired wiring pattern is obtained.
However, such manufacturing methods generally have the following disadvantages.
First, in the manufacturing method shown in (B) of FIG. 1, all copper foils necessary for the wiring are obtained by plating, and in such plating, the sufficient chemical as well as physical adhesion cannot be expected to occur in the interface between the finished copper foil and board. Consequently, with time lapse, the so called pattern separation, etc. are caused, whereby making it infeasible to produce the high quality circuit boards.
On the other hand, the manufacturing method with processing stages shown in (A) of FIG. 1 uses the two-side copper-clad laminate as the starting material for the board, therefore, it is advantageous in that the defect as mentioned above can be avoided. In other words, because this two-side copper-clad laminate generally has the characteristic of effecting the firm adhesion between the board and the copper foil, it is free of the problem such as the pattern separation. Also, it has the other merit that the low price, low cost circuit boards can be manufactured. Nevertheless, as is clearly understood from the Figure, since this manufacturing method uses the wiring pattern designed to have the double layer structure, there are the following disadvantages accompanying it. That is, first, a long time is required for etching, thereby making it unworkable to obtain the fine line, high precision patterning. Second, the waste in amount of copper used becomes large, resulting in the hindrance imposed on the cost reduction.
As the result, the latter manufacutring method is defective in that it cannot satisfactorily meet the need for forming the high density, high precision wiring pattern that is in specific demand today, and also that it is insufficient in capability to achieve the low cost and the resource saving.